remote sensing Article Multiple Remotely Sensed Lines of Evidence for a Depleting Seasonal Snowpack in the Near East Yeliz A. Yılmaz 1,*, Kristoffer Aalstad 2 and Omer L. Sen 1 1 Eurasia Institute of Earth Sciences, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey; [email protected] 2 Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo, Norway; [email protected] * Correspondence: [email protected]; Tel.: +90-506-346-6306 Received: 18 January 2019; Accepted: 21 February 2019; Published: 26 February 2019 Abstract: The snow-fed river basins of the Near East region are facing an urgent threat in the form of declining water resources. In this study, we analyzed several remote sensing products (optical, passive microwave, and gravimetric) and outputs of a meteorological reanalysis data set to understand the relationship between the terrestrial water storage anomalies and the mountain snowpack. The results from different satellite retrievals show a clear signal of a depletion of both water storage and the seasonal snowpack in four basins in the region. We find a strong reduction in terrestrial water storage over the Gravity Recovery and Climate Experiment (GRACE) observational period, particularly over the higher elevations. Snow-cover duration estimates from Moderate Resolution Imaging Spectroradiometer (MODIS) products point towards negative and significant trends up to one month per decade in the current era. These numbers are a clear indicator of the partial disappearance of the seasonal snow-cover in the region which has been projected to occur by the end of the century. The spatial patterns of changes in the snow-cover duration are positively correlated with both GRACE terrestrial water storage decline and peak snow water equivalent (SWE) depletion from the ERA5 reanalysis. Possible drivers of the snowpack depletion are a significant reduction in the snowfall ratio and an earlier snowmelt. A continued depletion of the montane snowpack in the Near East paints a bleak picture for future water availability in this water-stressed region. Keywords: snow cover; snow water equivalent; near east; water resources; GRACE; MODIS; AMSR-E; AMSR2; ERA5; euphrates; tigris; Kura-Araks; Coruh; Van Lake 1. Introduction The Near East region that encompasses the Fertile Crescent, often referred to as the cradle of civilization, has long been a center of agricultural activity. This has largely been due to a warm climate, nutrient-rich soil and the abundance of fresh water supplied by large snow-fed rivers such as the Tigris and the Euphrates. Presently, the waters of these rivers, being an essential resource for about 81 million people [1], are used extensively for industry, hydropower, and agriculture. Historically, prolonged droughts and a lack of water security have been major sources of conflict in this water-stressed region [2–5]. Currently, the water resources of the Near East are reported as highly vulnerable [6,7] with severe water scarcity throughout the year [8]. Worryingly, the sustainable use of this region’s water resources is threatened by the projected climate change related both to greenhouse forcing and land use change [6,9–11]. Several studies point out that the climate of the Near East region has been changing during recent decades (e.g., [12–16]). In the current era, the year 2007 was an important milestone for the region in terms of water resources. Strong observational evidence of a long-term drought in the Remote Sens. 2019, 11, 483; doi:10.3390/rs11050483 www.mdpi.com/journal/remotesensing Remote Sens. 2019, 11, 483 2 of 30 Fertile Crescent was shown for the 2007–2009 period in the study of Trigo et al. [12]. Türke¸set al. [15] reported significant changes in the temperature pattern over the coasts of the Black Sea and the eastern Anatolia where the headwaters of the Euphrates and Tigris basin (ETB) are located. They also found that the average precipitation increased by 1.9 mm per month from 1950–1980 to 1981–2010 for these areas. In their clustering analysis, they showed the formation of a new continental rainy summer precipitation pattern over northeastern Anatolia after 1980. For the same area, Gokmen [16] found an increasing trend of total annual precipitation over the period 1979–2010 up to 150 mm that agrees with the ERA-Interim/Land reanalysis data. The analysis of Gokmen [16] also indicated a significant decreasing trend in annual precipitation over the lower elevations in the ETB from station data and two reanalysis data sets. Several studies have projected a temperature increase pattern in the future over the Near East. Both global [17–19] and regional [9,10,20–23] climate modeling studies projected an absolute increase in the future temperature of around 1–9 ◦C (depending on the scenario and the model) over the region at the end of the 21st century. The direction and the magnitude of the projected changes in precipitation vary depending on the season and the subregion. Future simulations estimate a decrease in precipitation over the Near East region with an exception of winter months and the coasts of the Black Sea region [10,22–24]. In a statistical analysis, Ezber [25] used the outputs of future CORDEX simulations [26] to show a significant shift in precipitation seasonality over the Black Sea region and southeastern Anatolia. All these studies point towards the Near East being a region that is especially vulnerable to climate change. These findings indicate that the water resources of the region may be under threat in the future. Temperature has a direct effect on snow processes through modulating the phase of precipitation and the persistence of the snow once it is on the ground [27]. Both ground observations [13,14] and satellite products [28] show that increasing temperatures (up to 2 ◦C) have led to earlier spring snowmelt in the mountainous parts of the ETB. Sen et al. [13] investigated the changes in discharge timing in the ETB and teleconnection patterns, and concluded that earlier snowmelt typically occurs during the negative phase of the North Sea–Caspian pattern. The role of large-scale climate anomalies, particularly those related to La Niña, in forming droughts in the Middle East was studied by Barlow et al. [29]. They underlined the fact that projections point towards a drying and warming of the climate in the region, leading to a reduction in peak snow water equivalent (SWE) and earlier snowmelt. Yucel et al. [14] showed that streamflow timing in eastern Anatolia is already occurring over a week earlier compared to the climatological average for the period 1970–2010. By running regional climate simulations under a high emissions scenario, they projected that by the end of the century this shift would be as large as one month. The end of century projections also showed a considerable decrease in surface runoff for the Aras basin and the ETB, while a lower decline was found for the Coruh basin. In a hydrological modeling study, Bozkurt et al. [30] found similar results, projecting a decrease of 19–58% in annual streamflow in the ETB by the end of the century, corresponding to a temporal shift of streamflow timing of around 3–5 weeks. Ozdogan [31] evaluated the snow water availability in the whole ETB by forcing their model with the outputs of several regional climate simulations. Even though the results (decrease in SWE between 10 to 60%) show variations due to the different input data, they agreed on a significant decrease in SWE in the lower elevations (under 500 m) by the end of the century. A comprehensive analysis of snowmelt projections for different elevation bands was conducted by Onol et al. [10] for the ETB. Their five different regional climate model outputs projected that the seasonal snow cover would vanish in the lower regions (elevation between 1000 and 1500 m) at the end of the century. Moreover, snow in the middle and higher elevation regions (elevation > 1500 m) were projected to share the same destiny for the model outputs forced under various future scenarios. It is important to emphasize that other than reducing snowfall, drying has another important effect in that it may lead to an increase in snowpack ablation through sublimation [32]. These projections, including business as usual, paint a bleak picture for the future of the water resources of the Near East region. Remote Sens. 2019, 11, 483 3 of 30 Observations from space play an important role in monitoring changes in the state of the Earth’s surface [33–35] including the global terrestrial water storage [36] and the snowpack [37,38]. The globally available satellite retrievals of total terrestrial water storage (TWS) from the Gravity Recovery and Climate Experiment (GRACE) [39] are invaluable for diagnosing the state of water resources in areas such as the Near East where the availability of ground observations are both physically and politically limited. The GRACE TWS retrieval represents the water equivalent thickness which is the aggregate of all the water storage components within a pixel, namely soil moisture, surface water, groundwater, ice and snow (e.g., [36,40–43]). Voss et al. [40] used GRACE data to show the ongoing water loss in the Tigris-Euphrates-Western Iran region. They used the outputs of NASA’s Global Land Data Assimilation System (GLDAS) to decompose the GRACE TWS signal into its various storage components. Through this approach, they estimated a groundwater depletion of 17.3 ± 2.1 mm yr−1 for the period between 2003 and 2009. Longuevergne et al. [41] underlined that the groundwater contribution to the GRACE observation can be underestimated by half if the spatial distribution of the reservoirs is not considered in the ETB.